1. Field of the Invention
The present invention relates to a full color organic electroluminescent device and a method for fabricating the full color organic electroluminescent device, more particularly, to a full color organic electroluminescent device with reduced pixel misalignment.
2. Description of Related Art
Generally, an organic electroluminescent device comprises various layers including an anode and a cathode, a hole injection layer, a hole transport layer, an emitting layer, an electron transport layer and an electron injection layer. Organic electroluminescent devices are classified as polymeric organic EL (electroluminescent) devices and small molecular organic EL (electroluminescent) devices according to the materials used in making them. The respective layers are introduced by vacuum deposition for a small molecular organic EL device while a polymeric organic EL device is typically fabricated by a spin coating process.
The small molecular organic EL device is completed by laying up a multilayer organic film including a hole injection layer, a hole transport layer, an emitting layer, a hole inhibiting layer and an electron injection layer by deposition processes, and by finally depositing the cathode electrode.
A small molecular organic EL device is fabricated by an existing process by depositing the hole injection layer and the hole transport layer as common layers, depositing red, green and blue colors on the hole injection layer and the hole transport layer, patterning the red, green and blue colors on the hole injection layer and the hole transport layer using a shadow mask, sequentially depositing a hole inhibiting layer and an electron injection layer as common layers on the patterned red, green and blue colors, and depositing the cathode on the hole inhibiting layer and the electron injection layer.
Mass production of a small molecular organic EL device is difficult since such a full color device is fabricated by depositing the respective layers using masks. A fluorescent or phosphorescent device is fabricated by introducing the respective layers using vacuum deposition. Patents regarding full color devices include U.S. Pat. Nos. 6,310,360, 6,303,238 and 6,097,147.
A polymeric organic electroluminescent full color device is fabricated by patterning red, green and blue polymers. Such a polymeric organic electroluminescent device tends to have problems in its emission characteristics. In particular, it generally has lower emission efficiency and reduced lifespan when fabricated using ink jet technology or laser induced thermal imaging.
In order to apply a laser induced thermal imaging process to the fabrication of a full color polymeric organic electroluminescent device, at least a light source, a transfer film and a substrate are required. Light from the light source is absorbed by a light absorption layer of the transfer film so that the light absorbed into the light absorption layer of the transfer film is converted into thermal energy. A transfer layer forming material of the transfer film is transferred to the substrate by the thermal energy to form a desired image as disclosed in U.S. Pat. Nos. 5,220,348, 5,256,506, 5,278,023 and 5,308,737.
The laser induced thermal imaging can also be used to form patterns of emitting materials as disclosed in U.S. Pat. No. 5,998,085.
U.S. Pat. No. 5,937,272 relates to a method for forming an advanced patterned organic layer in a full color organic electroluminescent device in which a donor support is coated with a transferable organic electroluminescent material. The donor support is heated so that the organic electroluminescent material is transferred onto an object such as a recessed surface portion of the subpixels of a thin film transistor to form a colorized organic electroluminescent medium.
Therefore, any process for forming an emitting layer is restricted since fine patterning should be performed for each of the red, green and blue colors to fabricate a full color organic electroluminescent device.
FIG. 1 is a cross sectional view illustrating structure of a full color organic electroluminescent device according to prior art.
Referring to FIG. 1, the anode electrode is first patterned by depositing an anode electrode 12 on a substrate 10. An insulation film is applied to the substrate, and together with the anode electrode defines a pixel region. Then, a pixel region is defined by an insulation film 14. A hole injection layer 16 is coated over the red, green and blue pixel regions by such a method as vacuum deposition and a hole transport layer 18 is applied over the hole injection layer. Alternative, the hole injection layer and hole transport layer can be applied as a common layer. Red 100, green 200 and blue 300 are formed on an upper part of the hole transport layer 18 by vacuum deposition, spin coating or laser induced thermal imaging. Red, green and blue are patterned using a shadow mask when using vacuum deposition. However, it is not particularly necessary to use the shadow mask when using a laser induced thermal imaging method which transfers a desired part only.
A hole inhibition layer 20 is coated over the substrate, and an electron transport layer 22 is coated over the hole inhibition layer. Alternatively, the hole inhibition layer and electron transport layer can be coated as a common layer. Finally, a cathode electrode 24 as an upper electrode is laid up on the electron transport layer 22.
Prior art devices often have misalignment problems since the patterning process requires at least three deposition or transfer steps when forming the fine patterns of the red 100, green 200 and blue 300 on the pixel region. Furthermore, the hole inhibition layer is essentially required to be formed on an upper part of the emitting layer to prevent movement of the holes since the movement of holes is faster than the movement of electrons when using a phosphorescence emitting material as the emitting material for forming red, green and blue, that is, when using a host and a phosphorescence material as dopant in the pixel region.